The present invention relates to a multi-layer optical disk, particularly, to an optical disk formed into one sheet of a disk by piling up two layers of an information layer.
In recent years, such a demand that information recording media deal with a large capacity of information is increased, and in that case, too, a need for an optical disk of a large capacity is rising in order to be able to random-access and reproduce a target information.
As an optical disk capable of dealing with such information of a large capacity, there has been a proposal of a multi-layer disk or that of an optical disk formed as one sheet of a disk, especially, in which two information layers are piled up, that is, laminated.
Various kinds of the multi-layer disk have been proposed, and for one of them, there is a proposal of a multi-layer disk which is formed of a first information layer coated with a metal reflecting film and a second information layer coated with a semi-transparent reflection film whose spectroscopic characteristics such as a reflection factor, a transmission factor and the like have a wavelength dependency, and which is made to read out information from the first and the second information layers by using first and second reproduction lights of two different wavelengths (refer to a public bulletin for Japanese laid-open patent publication No 2-223030 and one for Japanese laid-open patent publication No 8-339574).
In the multi-layer optical disk having the first and the second information layers for carrying out a playback of the information by using the first and the second reproduction light, there is proposed a structure wherein, although the multi-layer optical disk is structured so that the playback is made possible by a reproduction light, for example, as the first reproduction light, which is within a wavelength range in a playback player for a conventional general purpose compact disk (CD) aimed exclusively for, for example, the playback, the playback is made possible with the same playback player of a conventional CD as for the other information layer.
The optical disk laminated with the first and the second information layers is, for example, as shown in a schematic cross-sectional view of one example thereof in FIG. 1, such that a first information layer 4f made up of a first information pit 2f formed on a surface of, for example, a first light transmission substrate 1f and a metal reflection film 3f coated and deposited thereon, and a second information layer 4s made up of a second information pit 2s formed on a surface of a second light transmission substrate is and a semi-transparent reflection film 3s coated and deposited thereon are bonded in such a way that an opposite side surface of a side where the first information layer 4f of the first light transmission substrate 1f is formed and a surface on a side where the second information layer 4s of the second light transmission substrate ls is formed are bonded with a transparent adhesive 5.
Then, as a first reproduction light Lf, a laser light with a wavelength range of 770 nm.about.830 nm of the reproduction light (hereafter, referred to as a first wavelength) in the playback player of the conventional general purpose, for example, reproduction-only compact disk (CD), is radiated from a rear side of the second light transmission substrate 1s to carry out the reproduction of the information from the first information layer 4f and as a second reproduction light Ls, a laser light shorter in wavelength than the former and a wavelength range of 615 nm.about.655 nm of a semi-conductor laser capable of being mass produced (hereafter, referred to as a second wavelength), is radiated to focus on the second information layer 4s, whereby the reproduction of the information from the second information layer 4s is carried out.
In order to obtain a reproduction signal of a high quality from the first information layer 4f by using such the optical disk,
(i) a reflection factor Rf1 of the metal reflecting film 3f against the first reproduction light Lf by the first wavelength must be high, PA1 (ii) a transmission factor Ts1 of the second information layer 4s relative to the light with the first wavelength must be as high as possible, that is, a light absorption factor As1 relative to the light with the first wavelength must be as low as possible, as well as its refection factor Rs1 against the light with the first wavelength must be as small as possible. PA1 (iii) a reflection factor R.sub.s2 of the semi-transparent film 3s against the second reproduction light with the second wavelength must be high. PA1 (iv) Therefore, from the above mentioned items (ii) and (iii), it is desirous that the semi-transparent reflecting film 3s of the second information layer 4s has a characteristic to the extent that Rs2-Rs1 is as large as possible (Rs.sub.2 is as larger than Rs.sub.1 as possible).
Also, in order to obtain a reproduction signal of a high quality from the second information layer 4s,
As a material to form the semi-transparent reflecting film 3s of the second information layer 4s, employment of silicon (Si) and a multi-layer structure with a dielectric substance are proposed.
As described in the above mentioned item (ii), because it is demanded that the light absorption factor of the light with the first wavelength is as small as possible, when the silicon is used, it is desirous that the silicon forms itself into a film as a thin film having excellent crystallization, but, due to a problem of thermal stability of the plastic substrate, there is some restrictions to forming the Si into the film having an excellent film quality on the light transmission substrate, particularly, a plastic substrate made up of poly-carbonate (PC) and the like, which is excellent in terms of mass productivity, costs and the like.
Also, when the dielectric substance is used for the multi-layer film structure, there is a problem that a process for manufacture becomes very complicated as compared with forming of a single layer film, thereby bringing a problem about high costs.
Also, as for the light transmission substrate, it is formed mainly of the poly-carbonate (PC) in consideration of a price, reliability, mass productivity and the like. In the case, however, there is a problem that double refraction is considerably large in comparison with glass. The double refraction is known to concentrate mainly on a very thin surface layer (a portion of nearly several ten .mu.m in thickness) of the substrate, but not dependent on the thickness of the substrate. Therefore, the problem of the double refraction does not depend on the thickness of the substrate but on how many times the target reproduction light passes through the surface thereof, and so, when two sheets of the light transmission substrates 1s and 1f as shown in FIG. 1, are used, the reproduction light for the first information layer 3f passes through the surface layers of both the two sheets of the light transmission substrates is and 1f and an influence of the double refraction becomes two times as much as a case of a single layer.
By the way, in order to be able to obtain a reproduction signal of a high quality in such the optical disk, it is desirable that the reflection factor against the light with the first wavelength is made more than 60%. This value can be made one index to obtain a high reproduction signal by a normalized value in a general purpose compact disk CD. Also, it is desirable that the reflection factor against the light with the second wavelength is 15%.about.30%. This value can be made one index to obtain the reproduction signal of a high quality by a normalized value in a two layer disk of a so-called DVD (Digital Versatile Disk)